The present invention relates to an apparatus for diagnosing semiconductor integrated circuits, and more particularly to an apparatus for detecting and measuring any void in a metal constituting interconnections of semiconductor integrated circuits.
For the semiconductor integrated circuits, requirements of improvements in high performance, high integration and minimization are no doubt being on the increase. Needless to say, establishment of good techniques for diagnosing semiconductor integrated circuits would also be extremely important in order to discriminate the perfect semiconductor integrated circuits from defective integrated circuits. Various techniques for diagnosing semiconductor integrated circuits have been known in the art to which the invention pertains.
One of the conventional techniques for diagnosing the semiconductor integrated circuits has been well known as an optical beam induced current method (an OBIC method). K. Haraguchi reported an improvement of the optical beam induced current method in its sensitivity and resolving power in "Detection of Defect Point and Failure Analysis in Semiconductor Device by OBIC" in which defects existing within a semiconductor material may be detectable by the optical beam induced current method, whose phenomenon will hereinafter be described with reference to FIGS. 1 to 3.
The optical beam induced current method is available for detecting any crystal defects in a semiconductor crystal structure. Phenomenon of the optical beam induced current method is as follows. A laser beam is irradiated on an intrinsic semiconductor in which the irradiated laser beam has such a wavelength as able to generate electron-hole pairs within the semiconductors. Such electrons and holes generated by the irradiation of the laser beams are drifted to move in the opposite directions to each other under an electric field since the intrinsic semiconductor is biased as illustrated in FIG. 1. Such drifts of individual carriers, for example, electrons and holes appear as an electrical drift current when the electric field in the semiconductor is greater than an electrostatic potential. The optical beam induced current is this current.
When the semiconductor includes some crystal defects, resistivity of the semiconductor at a point adjacent to the defects is higher than that at the remaining portion where the semiconductor has a perfect crystal structure. Such rise of the resistivity of the semiconductor around the defects causes some reduction of an amount of the optical beam induced current around the defects in the semiconductor. Irradiation of the laser beam on the semiconductor at a point adjacent to the crystal defects results in a reduction of the optical beam induced current. Thus, scanning of the laser beam irradiation on the semiconductor and a measurement of the optical beam induced current are able to sense positions of the crystal defects within the semiconductor. This optical beam induced current method is able to readily detect the crystal defects within the semiconductor even if the crystal defects exist at the inside of the semiconductor.
The above optical beam induced current method is available for diagnosing a semiconductor p-n junction which is biased in the reverse direction as illustrated in FIG. 2. When the p-n junction semiconductor is in reverse biased, a space charge region is formed at the junction surface. When a laser beam is irradiated on a surface of the space charge region in which the laser beam has such a wavelength or a frequency as able to generate electron-hole pairs or carriers, electrons and holes as carriers are drifted to move in the opposite directions to each other. This results in an appearance of an electrical drift current which represents an optical beam induced current. It would therefore be possible to sense the existence of the space charge region at the p-n junction by both scanning the irradiation of the laser beam across over semiconductors of opposite conductivity types and subsequent detection of the optical beam induced current.
As described above, the optical beam induced current method would be available for diagnosing the semiconductor material and thus be able to detect the crystal defects within the semiconductor materials only. This indicates that the optical beam induced current is not available for detecting any crystal imperfection of metals such as voids. Needless to say, the above prior art seems inapplicable to the detection of voids within the metals constituting the interconnections of the semiconductor integrated circuits.
Serious problems with diagnosing the semiconductor integrated circuit chip would surly be that the optical beam induced current is incapable of diagnosing the interconnections of metals in the semiconductor integrated circuits as its phenomenon is in utilizing the electron-hole pair generation, which appears in the semiconductor material, by irradiating the laser beam having such a wavelength as able to cause the electron-hole pair generation. For that reason, the above mentioned prior art method may be regarded as an imperfect diagnosing method for the semiconductor integrated circuits.
It have therefore been required to establish any method for diagnosing the interconnections of metals within the semiconductor integrated circuits namely any method for detecting voids existing in the metals constituting the interconnections of the semiconductor integrated circuits.
One of diagnosing methods for interconnections of the semiconductor integrated circuits is disclosed in 1990 IRPS (IEEE) pp. 200-208, W. Lee Smith at al. "DIRECT MEASUREMENT OF STRESS-INDUCED VOID GROWTH BY THERMAL WAVE MODULATED OPTICAL REFERENCE IMAGING". This method is applicable to diagnosing the interconnections of the semiconductor integrated circuit chips. This method is so called as a thermal wave modulated optical reflectance imaging method which permits efficient detections and measurements of any substantial voids in metals. When the laser beam is irradiated on a surface of void free metal having a perfect crystal structure, a thermal wave is generated at a beam spot of the metal surface and then radially and uniformly propagated into the inside of the metal as illustrated in FIG. 3. In contrast, the laser beam is irradiated on a surface of a metal involving some voids, an impedance of the propagation of the thermal waves appears around the void as illustrated in FIG. 4. This results in somewhat thermal accumulation around the void due to the impedance of the thermal wave propagation at a portion in the vicinity of the voids. Such thermal accumulation around the voids causes somewhat rise of resistivity of the metal adjacent to the voids. This reason is such that the resistivity of the metal is risen by rising a temperature of the metal which has been well known. Therefore, it could readily be understood that the measurement of any variation of the electrical resistivity of the metal by irradiation of the laser beam on the metal permits the detection of existence of the voids within the metal even if an invisible void exists at the inside of the substance of the metal.
The above described thermal wave modulated optical reflectance imaging system is illustrated in FIG. 5. The thermal wave modulated optical reflectance imaging system includes two lasers, for example, a HeNe probe laser for outputting a laser beam having a constant intensity and an Ar ion pump laser for outputting another laser beam having a variation in a sine wave of its intensity. The pump laser is connected through a modulator and a first beam expander to a first dichroic beam combiner. The first dichroic beam combiner is connected through an auto-focus to an objective lens. On the other hand, the probe laser is connected through a beam expander, a second dichroic beam combiner, a 1/4 wave plate to the first dichroic beam combiner. A photo-detector is connected through a HeNe filter to the second beam combiner. A pump laser beam is outputted from the pump laser to be transmitted through the modulator and the beam expander to the first beam combiner. A probe laser beam is outputted from the probe laser to be transmitted through the beam expander, the second beam combiner and the 1/4 wave plate to the first beam combiner. The pump laser beam and the probe laser beam are combined by the first beam combiner to be transmitted through the auto focus and the objective lens to a sample metal which is mounted on a sample holder and motion system. The laser beam is then reflected by the sample metal and revered through the auto focus to the first beam combiner where the reflected laser beam is turned to the second beam combiner through the auto focus. This sample holder and motion system is so designed as able to move in a plane for scanning the laser beam irradiation on an entire surface of the sample metal as illustrated in FIG. 5. The cause of the motion of the measured sample is as follows. As described above, the thermal wave modulated optical reflectance imaging apparatus is considerably large. It is therefore difficult to move such apparatus in the plane for scanning the laser beam irradiation on the entire surface of the sample metal. When the sample is such high density integrated circuit interconnections, it is much more difficult to realize the exact scanning of the laser beam irradiation on the interconnections of the metals in such semiconductor integrated circuits. For those reasons, the sample holder and motion system on which the sample metal is mounted is so moved in the plane as to realize the scanning of the laser beam irradiation on the interconnections of the semiconductor integrated circuits for diagnosing the crystal perfectiveness of the metals constituting the interconnections, while the thermal wave modulated optical reflectance imaging apparatus is secured at a predetermined position.
Although the above thermal wave modulated optical reflectance imaging apparatus is able to detect and measure any voids in the metals constituting the interconnections of the semiconductor integrated circuit, the above apparatus would be engaged with the following disadvantages.
The above thermal wave modulated optical reflectance imaging method may be considered as a costly and time-consuming method for detecting and measuring voids in the metals constituting the interconnections of the semiconductor integrated circuits. The expensive and time-consuming apparatus are due to the following reasons. As mentioned above, this conventional thermal wave modulated optical reflectance imaging apparatus employs the two laser devices, for example, one is the pump laser and another is the probe laser. For that reason, to accomplish the scanning operation for detecting and measuring any void within the metal, it is required to accomplish a precise alignment between pump and probe laser devices. Actually, it was however difficult to accomplish such a precise and exact alignment between the pump and probe laser devices during the scanning for diagnosing the interconnections of the semiconductor integrated circuits. Moreover, it was difficult for the conventional thermal wave modulated optical reflectance imaging apparatus to accomplish a high speed scanning for diagnosing the interconnections of the semiconductor integrated circuits since the sample holder and motion system for holding any sample such as a semiconductor integrated circuit chip would be moved on a plane while the apparatus is secured at a predetermined position.
Needless to say, it has been required to establish the exact diagnosing system for detecting and measuring any void within metals particularly interconnections of the semiconductor integrated circuits.